BCD Audio press announcement
London, 23rd November 2018
BCD Audio ALT2 enhancements 2018.
The BCD ALT-1 unit was introduced in 1993, in response to a need for a simple high-quality Broadcast microphone amplifier. The requirement was to use a 'TAKE' switch, where the unit would learn the gain required, and incorporate Compressor / Limiter / AGC technology so that the captured voice would be suitable for broadcast or recording without any further sound balancing. Oh; the unit had to have enough gain for a Coles lip-mic, but also provide Phantom power and be powerable from 12V DC, so simple plug-top power supplies or Radio car power could be used. Phantom power was to be switchable from a front panel switch, but lockable by a setup procedure. The gain setting and Phantom power were to be non-volatile, and the Compressor / Limiter / AGC optimised for Broadcast applications and output level.
The unit was successful, and was joined by the ALT-1S in 1997. This unit added a line-level balanced return path to the unit, that is unbalanced and level shifted inside the unit to a mini-jack, so low-cost PC style active Loudspeakers could be used in a Newsroom, to pick up the playout from the Programme audio matrix. The Mic-amplifier is identical.
Both units were reworked to fully solid-state in 2001. By then solid-state microphone amplifiers had good enough performance, so that the mic-transformer and gain relay could be eliminated.
The ALT-1 acquired an 'R' version, as many units were being installed inside other equipment, on rack-panels, etc. One switch on a Gallery TV control panel was enough to control an emergency floor-mic system, based on the ALT-1R.
The BCD-ALT-2R and BCD-ALT-2S units arrived in 2015. The ALT-2R has a 9pin D connector for the remote control and unbalanced output. The ALT-2S has the stereo balanced return feed to drive those PC style active Loudspeakers. A new surface-mount design simplified manufacture, and the audio path was kept the same.
In 2018, Danish Radio enquired about a version with less audio processing; if the recording via the ALT can be processed off-line, it makes sense to remove the Compressor and AGC functions, and raise the limiter threshold. BCD achieved this with changes to the internal firmware, and added a setup procedure where the user can decide on the operating mode. This procedure is similar to the Phantom power lock / unlock method: Hold down the 'TAKE' at power on to toggle between the original Compressor / Limiter/ AGC mode, and the new Limiter only mode. If the unit is powered up normally, the Compressor / Limiter / AGC LEDs indicate the mode for two seconds, before following the audio present. So original mode = all three LEDs on for two seconds, and limiter only mode = Limiter only LED for two seconds.
Adding the Limiter only mode exposed an issue with the 'TAKE' algorithm. This was not apparent before as the 2:1 compressor was always in circuit, and relates to the dynamics of speech, tone, and percussive signals.
The learning algorithm always knew what was the audio input level and output level, and worked with around 200 level samples over the 'TAKE' period. It always threw out levels that were too quiet, and ignored a few loud transient levels. But 5 seconds of tone at say -40dB would always produce a 0dB output, but 5 seconds of speech might be 4dB higher, as the peak-to-mean of speech is about 8dB, reduced to 4dB by the 2:1 compressor.
The new algorithm decides whether the character of the signal is like Tone, Speech, or Percussive. If the unit decides the source is similar to tone, the gain is calculated for 0dB output. If the unit decides the source is speech or percussive, 4dB less gain is selected. The overall effect is more predictable level setting under wider signal conditions.
All units shipped from the summer 2018 were shipped in original mode, but can be configured to either mode by following the setup procedure.